1. Field of the Invention
This invention relates to an improved method for producing a composite center electrode for a spark plug. The electrode is composed, for example, of a nickel alloy sheath with a copper core.
2. Description of the Prior Art
U.S. Pat. No. 3,144,576 granted Aug. 11, 1964 to Eugen Hagmaier et al. discloses a method for producing a composite center electrode by superposing a right circular cylindrical plate of a metal of good thermal conductivity, such as copper, on a right circular cylindrical plate of corrosion-resistant metal, such as nickel, within a die and extruding the plates therethrough to form a rod having a core of good thermal conductivity within a corrosion-resistant shell. The primary disadvantage of this method is that the length of the rod is limited because the core is formed with a taper which causes a correspondingly reduced thickness in walls of the shell as the metal plates are being extruded. This makes it impossible to assure uniform heat conductivity.
U.S. Pat. No. 3,548,472 granted Dec. 22, 1970 to Hisashi Urushiwara et al. discloses a method for producing a composite center electrode by subjecting a right circular cylindrical billet of corrosion-resistant metal to successive extrusions and drawings to form an elongated cup having a cavity extending therein, extruding a right circular cylindrical billet of a metal of a relatively higher thermal conductivity to form a core having a headed portion with a diameter equal to that of the elongated cup and a protruding portion of reduced diameter slightly less than that of the cavity, inserting the protruding portion of the core into the cavity and pressing the core therein to form an integral rod which is then subjected to heat diffusion. A composite center electrode is then formed by cold working the rod to form a head thereon. Both the elongated cup and the core are worked separately to dimensions substantially the same as their final dimensions before heat diffusion. Although this method produces a rod having a core of uniform diameter within a shell having walls of uniform thickness, the use of successive extrusions and drawings to form the elongated cup is uneconomical for mass production.
U.S. Pat. No. 3,857,145 granted Dec. 31, 1974 to Terumoto Yamaguchi et al. discloses a method for producing a composite center electrode by forming a cup from nickel or other corrosion-resistant metal, forming a cap having a headed portion with a diameter equal to the exterior diameter of the cup and a protruding portion of reduced diameter slightly less than that of the cup cavity of copper or of another metal of good thermal conductivity, inserting the protruding portion of the cap into the cavity of the cup, partially extruding the cup with the cap inserted therein through a die to form what the patent calls "a center electrode with a head". A rod having a core of uniform diameter within a shell having walls of uniform thickness is then cut from the "center electrode with a head" leaving, as scrap, the head and a part of the shell with a core therein. The head and the core are copper or the like while the shell part is nickel or the like. Finally, a composite center electrode is formed by cold working the rod to form a head thereon.